FIG. 1 is a simplified sectional diagram illustrating a Magnetic Tunnel Junction (MTJ) 100 of a conventional MRAM bit cell. In addition to the MTJ 100, the conventional MRAM bit cell has a transistor (not shown). With reference to FIG. 1, the MTJ 100 includes a fixed magnetic layer 102, a tunnel barrier layer 104, and a free magnetic layer 106. During operation of the MRAM bit cell, the magnetic moment (indicated by the unidirectional arrow) of the fixed magnetic layer 102 remains oriented in one direction. Conversely, the magnetic moment of the free magnetic layer 106 is designed to be either at 0° or 180° relative to the magnetic moment of the fixed magnetic layer 102. The orientation of the magnetic moment of the free magnetic layer 106 determines the state of the MRAM bit cell, as the resistance to a read current 108 through the MTJ 100 will be different when at 0° than at 180°.
FIG. 2 is a simplified plan diagram further illustrating the MTJ 100 of FIG. 1. As illustrated in FIG. 2, the MTJ 100 exhibits a generally oval shape. Two write lines 202, 204 are disposed substantially perpendicular to each other and are vertically arranged so that the MTJ 100 is between them. The write lines 202, 204 provide the current pulses that generate the magnetic field that orients the magnetic moment 210 of the free magnetic layer 106 in either the 0° or 180° state. In FIG. 2, the magnetic moment 210 of the free magnetic layer 106 is arbitrarily shown at the 0° state.
FIG. 3 is a simplified plan diagram illustrating another conventional MTJ 300 that exhibits a “stuck-at-mid” problem. The MTJ 300 has the same basic construction as the MTJ 100 of FIG. 1 and FIG. 2. The write lines 302, 304 of FIG. 3 are similar to the two write lines 202, 204 of FIG. 2, and the free magnetic layer 306 has a similar shape as the free magnetic layer 106 of FIG. 2. As illustrated in FIG. 3, vortices 308 that have unstable magnetic moments exist within the free magnetic layer 306 of the MTJ 300. The presence of the vortices 308 cause the predominant magnetic moment 3 10 of the free magnetic layer 306 to be aligned in a state that is neither 0° nor 180°. This causes the resistance of the MTJ 300 to be at an intermediate value between the “0” and “1” resistance values. The MTJ 300 is stuck because the current pulses that generate the write magnetic fields are unable to break the unstable magnetic moments of the vortices 308 and align them to the 0° or 180° state, hence the term “stuck-at-mid.” Because of the unstable vortices 308, it is difficult to determine whether the MTJ bit cell associated with MTJ 300 is to be read as a “0” or “1.” In an array of MTJ bit cells, there is typically a distribution of cells that exhibit this stuck-at-mid problem.
FIG. 4 is a flow diagram that illustrates processes included in a conventional method 400 of testing an assembled MRAM device. Method 400 includes a Hot Test (HT) 410, followed by a Burn-In (BI) test 420, and a Cold Test (CT) 430. After CT 430, additional Quality Assurance (QA) tests are performed at process 440. Assuming the MRAM device passes all tests 410-440 satisfactorily, it will be shipped at process 450. During Burn-In 420, a defective MRAM device is forced to fail by operating it for an extended period of time at voltages and temperatures that are elevated relative to maximum levels specified for the MRAM device.
Unfortunately, the elevated voltage and temperature conditions used during the Burn-In 420 can exacerbate the stuck-at-mid problem. The stuck-at-mid problem may also be caused by close proximity to external magnetic fields that can disturb the magnetic moment, or by elevated temperatures that occur during manufacturing of the MTJ. Example embodiments address these and other disadvantages of the conventional art.